This invention relates to and has among its objects the provision of a novel method and apparatus for controlling the matric potential of a liquid in a porous medium (matrix) particularly water in soil.
A liquid such as water can have different energy states. Water which is pure and free is arbitrarily defined as being at zero energy potential. Positive pressure on water increases the potential energy, and negative pressure or suction decreases the potential energy. In addition to pressure, gravitation and solutes can increase and decrease, respectively, the potential energy of water. The influence of gravitation is apparent only in saturated soils and a noticeable effect from solutes occurs only when their concentration is very high. In unsaturated soil, the dominant parameter of the water's potential energy is matric force of matrix suction which results from capillary and adsorptive forces. The capillary forces are dictated by the three dimensional geometry of the porous matrix and the adsorptive forces are controlled by the surfaces of the matrix material. Water will move in a matrix from a point of greater potential energy to a point of lower potential energy. The movement of water will cease when all water is of the same potential energy throughout the matrix, that is, when it is at equilibrium.
The control of the energy of a liquid in a matrix, such as water in soil, which results from matric forces, is important in the study of biological phenomena such as the amount of water needed for the movement of biological propagules, the effects of soil moisture on the availability of oxygen, the transfer of nutrients in soil water, and the transfer of heat sufficient for enzymatic reactions to take place in plant roots. For example, a controlled soil moisture matric potential is important in the study of the colonization of roots by bacteria.
Devices currently available for providing a constant matric potential to a rigid porous sample comprise a horizontal porous ceramic tension plate having an air entry value greater than the sample, and adapted for supporting the sample on a head of water, below. The pressure applied to the head of water is negative with respect to the atmospheric pressure, or the gas pressure otherwise applied to the sample from above. For the control of the matric potential of water in the sample, water potential must be equilibrated on both sides of the ceramic plate. The time required for equilibration is a function of the porosity of the plate, the porosity of the sample (e.g., soil), the initial difference between the matric potential on the opposite sides of the plate, and the maximum distance of any particle of the sample to the ceramic plate. Of course in the case of the planar tension plate device, the maximum distance of any particle to the plate is equal to the sample thickness.
The primary deficiency of this device relates to the fact that the equilibration time is an exponential function of the sample thickness. Therefore, as a practical matter, the matric potential can only be controlled for samples up to about 3 cm thick, because the time required for equilibration beyond that thickness becomes prohibitive. For example, in the laboratory study of biological phenomena in soil water, an equilibration period of no greater than 48 hours is desirable. An apparatus capable of maintaining a constant matric potential in thicker samples would be of paramount importance because a large percentage of all root associated microbial activity occurs in unsaturated soil at depths greater than 3 cm.